Drilling operators logically need as much information as possible about borehole and formation characteristics while drilling a well for safety and reserves calculations. If problems arise while drilling, minor interruptions may be expensive to overcome and, in some cases, pose a safety risk. Since current economic conditions provide little margin for error and cost, drilling operators have a strong incentive to fully understand downhole characteristics and avoid interruptions.
Gathering information from downhole can be challenging, particularly since the downhole environment is harsh, ever changing, and any downhole sensing system is subject to high temperature, shock, and vibration. In many wells, the depth of the well at which the sensors or transmission systems are positioned causes significant attenuation in the signals which are transmitted to the surface. If signals are lost or data becomes corrupted during transmission, the operator's reliance on that data may result in significant problems. Accordingly, many downhole conditions sensed while drilling a well have reliability concerns.
Typically, various types of sensors may be placed at a selected location along the bottom end of the drill string, and a mud pulser or other transmitter (e.g., electromagnetic), which are part of a measurement-while-drilling (MWD) system, is widely used in the oilfield industry to transmit and send signals to the surface. Signals from bottom hole sensors may be transmitted to the surface from various depths, but sensed conditions at a particular depth near the wellbore are generally assumed to remain substantially the same as when initially sensed. In many applications, this assumption is erroneous, and downhole sensed conditions at a selected depth change over time. In other applications, a downhole condition may not have changed, but the error rate in the transmitted signals does not provide high reliability that the sensed conditions are accurately determined. Updated sensed conditions are typically not available to the drilling operator, and accordingly most drilling operations unnecessarily incur higher risks and costs than necessary. For clarity, as formation changes rarely occur when drilling, the mud flow path is in constant change containing flow and transporting heterogeneous loads of formation cuttings.
A need remains for improved techniques to identify, measure, analyze, and adjust downhole conditions during drilling operations.